Processes for performing thermochemical reactions encompass a wide spectrum of reactions in which feedstocks are directly or indirectly heated to effect desirable endothermic reactions.
In the case of directly heated reactors, exothermic reactions effected in-situ provide the heat of reaction for the desired endothermic processes. Examples of such directly heated systems include partial oxidation and autothermal gasifiers. Although these systems can be used to gasify, for example, carbonaceous material, including biomass, the product gas is of low quality due to the presence of diluents, i.e., the products of the exothermic reactions.
Higher quality products can be obtained by the use of indirectly heated reactors. For example, several methods have been used for indirectly heated biomass gasification. One approach employs a conventional combustor with fire tubes immersed in a fluid-bed reactor. Flanigan et al., Proceedings of the 15th Biomass Thermochemical Conversion Contractor's Meeting, Atlanta, Ga., pp. 14-30 (1983). A second approach employs an auxiliary fluid-bed char combustor that heats sand in a separate bed. The hot sand is then used as the heat delivery medium in the primary fluid-bed gasification reactor. Feldman et al., Proceedings of the 15th Biomass Thermochemical Conversion Contractor's Meeting, Atlanta, Ga., pp. 31-90 (1983).
In the first approach, the large size of the combustor and heat exchange subsystem results in high cost relative to directly heated reactors. The major disadvantages of conventional fire-tube, indirectly heated reactors has always been the high cost arising from the size of the heat exchangers and the high-temperature materials required for construction of such heat exchangers. In addition, the large number of tubes required for heat exchange compromise the reactor bed fluidization. Thus, low heat release rates in the combustor and low heat transfer rates in the fire tubes limit the reactor's performance and economic viability.
In the second approach, char combustion to recoup the heating value of the char is difficult to achieve without long residence time and excess air, requiring an even larger (than the gasifier) combustor and further decreasing system efficiency. In addition, the size and complexity of the hot sand recirculation equipment and the cost of the additional fluid-bed char combustor both represent serious shortcomings.
As another example, prior art in steam reforming of heavy liquid hydrocarbons involves a number of fixed-bed and fluid-bed methods plagued by serious operational problems.
Most steam reformers for processing heavy liquid hydrocarbons to produce hydrogen-rich gas are autothermal, operating at high temperatures. This, however, compromises the product gas quality, due to the release of diluents (products of combustion in the product gas), particularly if the system is air blown.
This led to the development of two indirectly heated steam reformers worthy of note, one being the Total Hydrocarbon Reforming (THR) process (Tomita, High Temperature Processing Symposium, sponsored by KTI Company, Santa Barbara, Calif. (1979); Tomita et al., European Meeting of Chemical Engineering, 18th Chemical Congress and Exhibition, Frankfurt Germany (1976)), the other being a catalytic fluidized steam-reforming process ((hereinafter the "French process") Bulletin from Societe de la Grande Paroisse, (1973)).
In the THR process, hydrogen is produced by the reaction of steam with the heavy liquid hydrocarbon in a fixed-bed tubular reactor. This process is catalytic and is reported to accept a range of feedstocks including naphtha and crude oil without any feedstock pretreatment. The THR process employs a catalyst which operates in the presence of sulfur.
The primary catalyst is called T-12 and is a silica-free, calcium aluminate based catalyst. Because the steam reforming activity of this catalyst is lower than that of conventional nickel catalysts, the required reaction temperature is higher. Thus, for a heavy feedstock such as Iranian heavy crude, inlet temperatures are on the order of 1652.degree. F. and exit temperatures as high as 183220 F., giving rise to serious heat transfer and tube material problems. It should also be noted that it was necessary to develop a complex new feed system to control the heavy fuel vaporization and vigorous mixing with steam to avoid cracking and soot formation at the reformer tube inlet.
Because the Ni-free T-12 catalyst is not sufficiently active to convert all the hydrocarbons to synthesis gas, the exit gas inevitably contains a high level of methane, particularly with heavy feedstocks. To solve this problem of hydrocarbon breakthrough, a Ni-containing catalyst (T-48) is used at the end of the T-12 calcium aluminate fixed-bed. The T-48 nickel catalyst, which is adjacent and upstream from the T-12 catalyst, is sulfur tolerant in this process because it is operated at high temperatures, usually 1650.degree. F., and in the presence of substantial amounts of H.sub.2. For steam reforming of crude oil, the THR process is more costly than conventional naphtha steam reforming.
It is apparent to those familiar with the steam reforming art that processing of heavy hydrocarbons poses unique problems due to the existence of aromatic constituents contained within the heavy hydrocarbons which are particularly prone to forming carbonaceous deposits or soot on catalytic substrates. In the THR process, the primary catalyst is disposed within a fixed bed tubular reactor. The deposition of carbonaceous deposits in such fixed bed tubular reactors results in occlusion of the catalyst void volumes. In the fixed bed configuration, the process of deposition and occlusion is progressive, leading to excessive pressure drop within the tubular reactor and necessitating shutdown. Thus, since deposit formation cannot be tolerated in fixed bed reactors, process conditions must be established to avoid or minimize its occurrence. This generally requires the use of high steam to carbon ratios which enhances the rate of carbon gasification relative to the rate of carbonaceous deposit formulation. However, high steam to carbon ratios are detrimental to the thermal efficiency of the process.
In the case of the French process, developed at the Societe de la Grande Paroisse, a fluid-bed reactor was employed. The reactor was developed to process heavy, sulfur-containing feedstocks (e.g., fuel oil) to hydrogen with no desulfurization and minimum carbon formation. In this process, water and hydrocarbon are fed into a fluidized-bed of nickel-containing catalyst which is maintained isothermally at 1472-1690.degree. F. The fluidized bed operation permits operation at low steam/carbon ratios. However, the heavier feedstocks cause some hydrocarbon breakthrough. Moreover, having a nickel-containing catalyst in the fluid-bed process is not desirable for two reasons. The first reason is that attrition in the bed causes loss of the expensive nickel-containing catalyst. The second reason is extensive soot formation and sulfur poisoning of the nickel in the catalyst, which is encountered when processing heavy liquids which are soot formation prone and contain a significant amount of sulfur (Number 4 and 6 fuel oils).
In addition, certain reactant materials present unique challenges to reactor, process, and system design. Black liquor, the by-product of pulping processes, generally contains biomass-derived lignins and inorganic sodium and, in some instances such as in the case of the Kraft liquor, sulfur process chemicals. The economics of the process dictate the need for recovering the process chemicals and energy values of the black liquor.
The Kraft black liquor recovery process, for example, must provide a means for conserving and/or regenerating sulfur in the sodium sulfide form. This is currently being accomplished using a Tomlinson recovery furnace, wherein black liquor is combusted and the inorganic sulfate chemicals are reduced by reaction with carbon in a molten smelt bed at the bottom of the furnace. Although the Tomlinson furnace has been widely employed in the Kraft paper industry for several decades, it possesses significant deficiencies, including safety hazards, i.e., smelt-water explosions, corrosion and undesirable environmental emissions. In addition, Tomlinson furnaces represent a significant fraction of the total capital expenditure for a modern mill. When mill expansions are contemplated, there exists little opportunity for incremental plant capacity expansion because recovery boilers are economically viable only in large capacities.
For these reasons, the paper industry has sought new technology alternatives to the Tomlinson recovery boilers. Gasification of black liquor can be accomplished autothermally; however, this approach results in product gas of low heating value and in most instances, such autothermal gasifiers produce molten smelt. More importantly, since the Kraft chemicals must be recovered in a reduced state, direct exposure of black liquor to oxidants, e.g., in partial oxidation and autothermal processes is generally undesirable. Others have demonstrated autothermal gasification of black liquor in a molten salt reactor. Although reduction of Kraft chemicals by carbon contained in the molten salt has been established in an autothermal gasifier, this route suffers from many of the same difficulties which plague the Tomlinson furnace technology, including smelt production, corrosion problems, explosion hazard, high capital cost, and low system efficiency.
Thus, there is a need for a black liquor recovery process that obviates the need for molten smelt handling, provides high reliability and safety, high thermal efficiency, low cost and is amenable to modular system configurations to support incremental mill expansion.
For a variety of applications, there is a need for both new reactor technology for indirectly heated thermochemical processes, and for the various endothermic processes, optimization of the reactions and process parameters to maximize the benefits. Needs for new indirectly heated thermochemical reactor technology and processes exist in a very wide spectrum of end-use applications, including, e.g., mild gasification of coal, steam gasification of coal and peat, thermal cracking of chemicals, industrial and municipal waste thermochemical processing, gasification of energy-bearing waste streams from food processing plants, recovery of useful fuel forms from oil shale and oil and tar sands, detoxification of and energy recovery from hazardous waste materials, and generally effecting endothermic reactions in chemical processes for production of desired chemicals.
Advantages may be gained in heat release and heat transfer rates by the utilization of pulsating combustors. The combustion intensity of pulse combustors is high. Thus, for a given heat output, the combustion chamber is relatively small. Further, because the combustion products are driven by combustion-induced oscillations, the boundary layer resistance to heat transfer, from the flue gas to the inner wall of the fire tube (resonance tube), is reduced and the heat exchange surfaces may be correspondingly smaller for a specific output. For example, U.S. Pat. No. 4,655,146 refers to a reactor for performing high temperature reactions such as melting, heat treating and incineration. The reactor comprises a combustion chamber, or extension thereof, tuned to resonate and thereby achieve efficient combustion. Fuel and the reaction material are fed into and undergo reaction within the chamber. U.S. Pat. No. 3,606,867 refers to a pulsating combustion system for producing a high temperature and pressure gas stream to impinge on objects for heat treatment. U.S. Pat. No. 2,937,500 refers to resonant jet engines in combination with heat exchange equipment, which equipment is characterized by a sonically augmented rate of heat transfer for use in heating the air supply to the engine. These patents are incorporated herein by way of reference. None of these patents or any of the aforementioned thermochemical processes suggest the use of pulsating combustion in connection with an indirectly heated fluid bed reactor.
The present invention overcomes the deficiencies of the currently used indirectly heated reactors by utilizing a single or, preferably, multiple resonant tube(s) of a pulsating combustor emanating from the same combustion chamber as the in-bed heat exchanger, wherein the velocity and pressure oscillations of the combustion gases and the intense acoustic field radiated by the multiple resonance tubes into the reactor bed enhance rates of heat release, heat and mass transfer and, ultimately, the rates of reaction in the bed.